1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) mobile communication system. More particularly, this invention relates to a CDMA mobile communication system of automatically changing a frequency and method thereof for continuously maintaining a call service for a terminal which belongs to a cell covered by a base transceiver station even when devices constituting the base transceiver station have errors thereby to abnormally operate or when the device having an error is restored to a normal state while the base transceiver station is performing multi-frequency assignment operation.
2. Discussion of Related Art
FIG. 1 is a block diagram showing configuration of a CDMA mobile communication system in hardware.
This CDMA mobile communication system comprises a mobile station 1 which is a terminal of a subscriber, such as a car phone and cellular phone, a base transceiver station 2 constructed in each cell for connecting the mobile station 1 to a base station controller 3 thus allowing transmission and reception of various signal information through a data link, and the base station controller 3 for processing incoming/outgoing traffic channels and calls of a switching center 4.
In the general CDMA mobile communication system, a frequency of about 800 MHz having a bandwidth of 10 MHz is assigned to a base transceiver station, and this frequency is divided into about 8 frequencies having a bandwidth of 1.25 MHz for a control. In the multi-frequency assignment operation of the mobile communication system, in case that a device has an error in an abnormal state and is not capable of performing its function, terminals which use the frequency charged by the device having the error cannot be given a call service. When a cell is defined as an area controlled by a base transceiver station and this cell is divided into three sectors, such as an alpha, beta, and gamma as shown in FIG. 2, using a directional antenna, each sector uses multiple frequencies (for example, eight frequencies) in common, with interference of one another being minimized using the directional antenna.
There are many devices in a base transceiver station. A radio frequency card (RFC) converts an intermediate frequency (IF) signal of 4.590 MHz into a signal of an ultrahigh frequency (UHF) band. One RFC is installed for one sector and includes multiple parts performing the above function for each frequency. An analog common card (ACC) receives three timing signals and supplies them to the remaining parts of a digital shelf. One ACC is installed for one sector and has multiple parts performing the above function for each frequency. A sector interface card (SIC) combines baseband forward signals, up-converts a combined result into the IF frequency, and supplies timing. One SIC performs the above function for each frequency of each sector.
In the multi-frequency assignment operation of a base transceiver station, in case that an error occurs in a part for a specific frequency in the RFC of a specific sector thus the part is in an abnormal state, terminals using the relevant frequency and sector cannot be given a call service. Similarly, in case that the ACC has an error and malfunction, terminals using the relevant frequency and sector cannot be given a call service. When a part of the SIC for a specific frequency has an error, terminals using the relevant frequency cannot be given a call service regardless of the sector.
When making a primary call, a terminal receives overhead information from a base transceiver station using a fundamental frequency, has its own mobile identification number (MIN), and determines which frequency it will use according to a hash function. When an error occurs in a part of the RFC or ACC corresponding to the fundamental frequency resulting in a malfunction, the overhead information cannot be received from the base transceiver station, whereby the terminal cannot use other normal frequencies as well as the fundamental frequency. When an error occurs in a part of the SIC for the fundamental frequency, the terminals within the relevant cell cannot be given the call service although other parts for other frequencies are in normal state.
In other words, there is a drawback in the conventional CDMA system that all terminals in a cell covered by a base transceiver station cannot take a call service when an error occurs in a part charging a fundamental frequency in a device constituting the base transceiver station.